Quantum-mechanical tunnelling allows electrons to travel across a nanoscale gap between two conducting electrodes. The tunnelling current thus produced can excite surface plasmons (the collective oscillations of electrons on the surface of metallic nanostructures).

During tunnelling, most electrons tunnel elastically – that is, they maintain their energy – and tunnel junctions can be used to “harvest” these hot electrons. “The good thing for our experiments is that tunnelling is extremely sensitive to any changes in the properties of the tunnel junctions (even an atomic-level change in the junctions can dramatically affect the tunnelling process),” explains team member Pan Wang. “In our case, chemical reactions in the tunnel junctions result in a significant change in the light emission intensity as well as the tunnelling current in the plasmonic nanorod metamaterials we studied. The reaction kinetics can thus be monitored in real time by monitoring the change in light emission and tunnelling current through these nanostructures.”

Nanoreactors or lab-on-a-chip devices

“The structures might be used as nanoreactors or lab-on-a-chip devices to monitor and better understand chemical reactions,” he tells nanotechweb.org. “They might also be used as highly sensitive gas or molecule sensors since the tunnel junctions can be designed to bind or react with a target molecule or molecules. We would detect the molecules in question by monitoring the change in light emission or tunnelling current, as mentioned.”

The process of generating plasmonic and photonic signals from tunnelling electrons might also be exploited to develop integrated light sources for optical interconnects in photonic chips, he adds. “At the moment, semiconductor lasers are usually employed as the light sources for such interconnects, but these are bulky and cannot be fabricated using CMOS-compatible processes. In our experiments, we can generate light or plasmons by applying a voltage across a nanometre scale gap between two metal electrodes so that the set-up can be directly used as a light or plasmon source.

Optical data transfer applications

“This structure, which is simple and small and which can be fabricated using CMOS-compatible process, can also be used to optically transmit information between or within photonic chips. And that is not all: the same structure can be used to a build a photonic waveguide or integrated in another waveguide material (for example, one made from silicon), so the generated light can be directly coupled to the interconnect without the need for additional couplers, adds Wang.

The researchers, led by Anatoly Zayats say that they will now be looking into the interaction between light/plasmons and the tunnelling effect in more detail. “There are many potential discoveries here and we are looking forward to new real-life applications made possible by our electrically driven metamaterial system.”

Full details of the research are published in Nature Nanotechnology doi:10.1038/s41565-017-0017-7.